The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins

Abstract

The Neoproterozoic shoshonitic and mildly alkaline bimodal volcanism of Southernmost Brazil is represented by rock assemblages associated to sedimentary successions, deposited in strike-slip basins formed at the post-collisional stages of the Brasilian/Pan-African orogenic cycle. The best-preserved volcanosedimentary associations occur in the Camaquã and Campo Alegre Basins, respectively in the Sul-riograndense and Catarinense Shields and are outside the main shear belts or overlying the unaffected basement areas. These basins are characterized by alternation of volcanic cycles and siliciclastic sedimentation developed dominantly on a continental setting under subaerial conditions. This volcanism and the coeval plutonism evolved from high-K tholeiitic and calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, and its evolution were developed during at least 60 Ma. The compositional variation and evolution of post-collisional magmatism in southern Brazil are interpreted as the result mainly of melting of a heterogeneous mantle source, which includes garnet-phlogopite-bearing peridotites, veined-peridotites with abundant hydrated phases, such as amphibole, apatite and phlogopite, and eventually with the addition of an asthenospheric component. The subduction-related metasomatic character of post-collisional magmatism mantle sources in southern Brazil is put in evidence by Nb-negative anomalies and isotope features typical of EM1 sources.

Zircon U-Pb geochronology, Sm-Nd and Pb-Pb isotope systematics of Ediacaran post-collisional high-silica Acampamento Velho volcanism at the Tupanci area, NW of the Sul-Rio-Grandense Shield, Brazil

“…8B). These characteristics were also identified in other occurrences of the AVF at the SRGS (Sommer et al 2005b(Sommer et al , 2006, where low and high-Ti groups of basalts and rhyolites were identified. The magmatic source of these groups and their petrogenetic correlation are still not well constrained.…”

“…The CB is a retroarc basin that includes a set of preserved sedimentary and volcanogenic units, which were deposited between 620 and 535 Ma (Brito Neves & Cordani 1991, Gresse et al 1996, Brito Neves et al 1999, Chemale Jr. 2000, Fragoso-César et al 2000, Paim et al 2000, Sommer et al 2006, Janikian et al 2008. The basement of the CB is heterogeneous and ranges from paleoproterozoic granulitic complex to associations of metamorphic and deformed igneous rocks related to the BrasilianoPan-African orogeny (Paim et al 2000, Lima et al 2007.…”

“…The magmatic source of these rocks is probably related to partial melting of a lithospheric mantle previously chemically affected by a subduction process (Nardi & Bonin 1991, Wildner et al 1999, 2002, Nardi & Lima 2000, Almeida et al 2005, Sommer et al 2006. Sommer et al (2005aSommer et al ( , 2005bSommer et al ( , 2006 suggest that these volcanic cycles represent preserved portions of a typical evolutionary sequence from a post-collisional magmatism (from Liégeois 1998), with shoshonitic affinity rocks in the early stages, chemically affected by subduction-related processes, followed by silica-saturated bimodal volcanic sequences, with sodic-alkaline affinity and finally by tholeitic rocks. of the SRGS (Fig.…”

“…Analysis of two rhyolitic pebbles from the conglomeratic fluvial deposits of the basal portion of the AVF provided zircon U-Pb ages of 579 ± 13 Ma and 569 ± 2.4 Ma (Janikian et al 2012). However, these authors proposed the individualization of a new younger acid volcanic unit related to reactivation of extensional structures identified in Taquarembó Plateau, for which the crystallization age, obtained by U-Pb LA-ICP-MS method in lapilli-tuffs previously considered as part of the AVF, is 544.2 ± 5.5 Ma (Wildner et al 1999, 2002, Sommer et al 2006. This was correlated with the U-Pb zircon SHRIMP age of 549.3 ± 5 Ma obtained by Sommer et al (2005a) in hypabyssal rhyolites of the Ramada Plateau.…”

“…This episode is mainly exposed in volcanic plateaus and ridges composed mostly of acid effusive and pyroclastic rocks with minor intermediate and basic components (Sommer et al 2005a, Wildner et al 2002, Almeida et al 2002. Eruptive periods were initiated with explosive episodes and closed by effusive events (Sommer et al 2006). The best exposures of this volcanic sequence are in the Ramada Plateau (Vila Nova do Sul City), Taquarembó Plateau (Dom Pedrito City), Bugio and Perau Hills (Caçapava do Sul area, Santa Bárbara Hill), besides the Tupanci region, the northernmost exposure of this volcanism in the SRGS.…”